A container of this type is described in European application No. 337,860, in the name of the present applicant.
With this type of container, typically using as refrigerant solid phase CO.sub.2 (dry ice or carbon dioxide snow), penetration of heat across the heat insulation of the isothermal casing is compensated by sublimation of the dry ice, thereby liberating cold gas. The shield allows the upper part of the load in the lower chamber to be largely isolated from the cold source, and promotes circulation of cold gas to the periphery of at least the upper part of the lower chamber. In practice, the load must in no case be allowed to contact or become flush with the thermal shield, and it is therefore necessary to provide a clearance, typically at least 2 cm, between the shield and the upper end of the load in the lower chamber. This spacing principally promotes circulation of cold gas immediately beneath the thermal screen shield, which affects the natural convection conditions of the cold gas in the lower chamber.